1. Field of the Invention
The invention relates to a vehicle braking system and, more particularly, to improvement of a replacement control of replacing one of the braking torque provided by a rotary machine and the braking torque provided by a mechanical brake with the other one.
2. Description of Related Art
There is a known braking control apparatus for a vehicle which includes (a) a rotary machine that functions at least as an electricity generator and that generates braking torque based on regenerative torque, (b) and an electric brake control apparatus that electrically controls the braking torque of a mechanical brake that is provided for a wheel of the vehicle, wherein (c) one of the braking torque provided by the rotary machine and the braking torque provided by the mechanical brakes are replaced with the other one while a target braking torque is maintained. An example of this type of braking control apparatus is an apparatus described in Japanese Patent Application Publication No. 2007-276655 (JP 2007-276655 A) which is equipped with a hydraulic brake as a mechanical brake. When the vehicle speed becomes less than or equal to a predetermined speed immediately before the vehicle stops, the vehicle braking control apparatus linearly lowers the braking torque provided by the rotary machine according to decline in the vehicle speed and, at the same time, linearly raises the braking torque provided by the hydraulic brake.
However, in general, the response of the mechanical brake is lower than the response of the braking torque based on the regenerative control of the rotary machine. This difference in response sometimes results in a replacement shock in which the braking torque of the entire vehicle instantaneously declines. FIG. 6 shows a case where the braking torque provided by the rotary machine (hereinafter, referred to as “MG torque”) is replaced with the braking torque provided by the mechanical brake (hereinafter, referred to as “ECB torque”) in the range of vehicle speed of Vb to Va during deceleration of the vehicle during which the demanded braking torque T1 (hereinafter, referred to as “target braking torque”) demanded by a driver's brake operation is substantially constant. While the ECB torque command value is linearly increased corresponding to the MG torque, the actual ECB torque changes as shown by a two-dot chain line due to response lag. As a result, the combined torque obtained by combining the MG torque and the ECB torque (which corresponds to the braking torque of the vehicle) instantaneously drops as shown by a dotted line to cause a shock. The response lag of the ECB torque depends on, for example, the oil temperature of the hydraulic oil, variations in the manufacturing precisions of various valves, etc. Therefore, the response lag of the ECB torque is not always same, so that it is difficult to achieve adaptation of the combined torque even if the changing of the MG torque is delayed by taking the response lag of the ECB torque into account.
Although in JP 2007-276655 A, the operation speed of an electric oil pump that produces the oil pressure of the hydraulic brake is raised prior to the replacement control, so that a sufficient amount of oil is secured from the commencement of the replacement control. However, this publication does not take into account the response lag due to flow resistance at all, leaving room for improvement.
Furthermore, although in JP 2007-276655 A, the braking torque provided by the rotary machine is replaced with the braking torque provided by the mechanical brake, there also is possibility of replacement shock resulting from the response lag of the mechanical brake in the case where the braking torque provided by the mechanical brake is replaced with the braking torque provided by the rotary machine.